A root in The Etymologies of the 1930s glossed “round, about”, with various derivatives like N. o “about, concerning” and ᴹQ. osto/N. ost “city, town with wall round” (Ety/OS). Tolkien also considered variant forms ᴹ√OD and ᴹ√OTH, the latter glossed “fort” (EtyAC/OS). This root in The Etymologies is a later iteration of unglossed ᴱ√OSO from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. ossa “wall and moat” and ᴱQ. ostar “township” (QL/71). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. ost “enclosure, yard; town” and G. osta- “to surround with walls, build up; fortify, protect” (GL/63). This early root probably meant something like “✱enclosure”.
Neo-Eldarin: For purposes of Neo-Eldarin, ᴹ√OS “round, about” is too useful to discard, and I prefer it over the later root √SOT, though √(O)S-OT might be salvaged as an extension of √OS.